In 1850 the admission of California gave the free states a majority of two votes in the Senate. This majority was increased to four by the admission of Minnesota in 1858, and to six by the admission of Oregon in 1859. The control of Congress had slipped forever from the grasp of the slave states.

The tremendous increase in manufacturing, in farming, and in trading brought about a great increase in foreign commerce. This in turn led to the building up of great cities in the North and the West. These were New York and Chicago; and they grew rapidly because they formed the two ends of the line of communication between the East and the West by the Mohawk Valley. New York now contained over eight hundred thousand inhabitants. It had more people within its limits than lived in the whole state of South Carolina. The most rapid growth was seen in the case of Chicago. In 1840 there were only five thousand people in that city; it now contained one hundred and nine thousand inhabitants. Cincinnati and St. Louis, each with one hundred and sixty thousand, were still the largest cities of the West, and St. Louis was the largest city in any slave state. New Orleans, with nearly as many people as St. Louis, was the only large city in the South.